In gas turbines it is desirable to reduce gas leakage around the turbine blades in order to improve the efficiency of the turbine. This can be achieved by surrounding each array of turbine blades with a ring of abradable honeycomb material. As the turbine rotates the tips of the turbine blades cut a path through the abradable material, so ensuring that only a very small gap is left between the turbine blade tips and the surface of the abradable material. Since this gap is very small, leakage is restricted.
Unfortunately the abradable material tends to erode slowly in the extreme environment found within the turbine. As a result the abradable material must be replaced regularly. In order to make replacement simple the abradable material is supported by metal shroud liners. These shroud liners are in turn attached to the structural casing of the turbine. Furthermore the shroud liners are circumferentially segmented to make assembly simpler, allow individual areas of the lining to be replaced, and to accommodate better any distortions caused by the extreme temperatures within the turbine.
It is necessary to attach the shroud segments to the structural casing so that they are held accurately relative to the blade tips. This is important since any movement is likely to increase the clearance at the blade tips, so increasing leakage. The mounting is either directly from the casing, from the stationary nozzle guide vane assemblies which precede and follow the turbine rotor and are themselves fixed to the casing, or from a combination of both. A conventional arrangement is to have accurately machined circumferential slots or grooves into which mating lugs locate. This provides accurate fixed location of the segments.
Shrouded turbine blades can be employed to further reduce leakage around the blades. By using a shrouded blade a seal can be produced between the blade shroud and the abradable surface of the shroud segment. The seal further reduces leakage past the blade tip. Typically a fin seal arrangement is used. A step can be provided between successive fins to improve the seal effectiveness. A corresponding step is also provided on the profile of the abradable honeycomb material on the segmented shroud liner. The profiling and the cooperation with the stepped fins upon shrouded blades makes accurate assembly complex. Such arrangements generally require that the shroud segments of the shroud liner are fitted, at least partially, into the casing before, and without, the turbine rotor assembly with which they are associated being fitted. If the segments are fitted with the turbine rotor already installed then an excessive clearance between the shroud segment and blade shroud would be required to allow for assembly. This is particularly the case when the rear fins of the fin seal are of a larger radius than the forward fins. This excessive clearance would produce an increased leakage over the turbine blade tips and therefore a consequent performance loss.
Other considerations may require that the turbine rotor has to be fitted into the casing before the shroud segments are fitted. This can be the case if, for example, the turbine rotor of one stage of the gas turbine engine has to be assembled and balanced with another associated component of the engine. To ensure the components remain in balance the resultant rotating assembly has to be fitted as single unit. In these cases a stepped shroud liner and shrouded blades are generally not used, and thus the performance improvement is not realised.